Analysis of the terminal portion of the QRS of the ECG and detection of late potential (LP) by signal averaging (SAE) is a noninvasive method for identifying patients who are prone to develop ventricular tachycardia (VT) and are therefore at risk for sudden cardiac death. We have earlier shown that there was a and correlation between the presence of LP and inducibility of VT in patients with hypertrophic cardiomyopathy (HCM). Amiodarone is a patient antiarrhythmic drug. The purpose of the present investigation was to determine whether changes in SAE predicted efficacy and proarrhythmic potential in 14 HCM patients treated with amiodarone. Amiodarone increased the incidence of LP. This was associated with the greater ease with which VT was induced during programmed electrical stimulation. Thus SAE may identify HCM patients in whom amiodarone is proarrhythmic and may be at increased risk for sudden cardiac death.